1. Field of the Invention
This invention relates to a loop-seizing and dial pulsing circuit having a high impedance and of a type used in telephone trunk circuits.
2. Description of the Prior Art
Trunk circuits are commonly employed in telephone systems to provide a two-way signaling, supervision and transmission path between a central office connected to the trunk circuit and a distant office connected to the trunk circuit via a transmission facility.
For example, one type of trunk circuit used to provide two-way signaling and supervision between an office of the step-by-step type with a distant office operates as follows. For an incoming call from the distant office, the trunk is seized when the distant office closes the loop on the trunk tip and ring leads generally designated as T and R. The trunk circuit detects the loop closure and repeats the seizure by enabling a loop seizure circuit which in turn provides a current limited loop closure toward the step-by-step office on tip and ring leads generally designated as T1 and R1 which connect to the step-by-step office. Wink start may be returned to the distant office as a loop reversal on the leads T and R. Dial pulses transmitted from the distant office will be detected by a loop detector circuit which in turn controls the loop-seizing circuit to repeat the pulses to the step-by-step office. The trunk circuit recognizes answer supervision from the step-by-step office as a loop reversal on the leads T1 and R1 and repeats the supervision with a loop reversal on leads T and R toward the distant office. A transformer coupled voice path is then established and conversation between the calling and called parties may commence.
In such a trunk circuit as described above, the loop-seizing circuit must be arranged to operate with opposite polarities on its inputs to provide two-way signaling capability. Furthermore, it must provide a current limited direct current path between its input leads.
In one prior such trunk circuit, the loop-seizing circuit comprises a full wave diode bridge across the tip and ring leads. A conventional two transistor constant current source utilizing a resistor to provide base current bias is connected across the diode bridge. An optical isolator comprising a phototransistor connected in series with the above mentioned bias resistor and a light emitting diode coupled to the phototransistor is used to control the constant current source. A second opto-isolator having its light emitting diode connected between the diode bridge and the ring lead is used to detect loop reversals.
One problem with the prior art loop-seizing circuit is that a relatively low impedance path is provided for A.C. signals. For example, in such a prior art circuit employing a base biasing resistor insertion loss has been measured at between 0.2 db and 0.5 db.
A further problem with the prior art circuit is that a high gain transistor must be used because base current will decrease as loop resistance increases.